Single Event page based on scope and use of PLACEHOLDER in PHP template file

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You need to move your template files – instead of

/wp-content/themes/my-child-theme/plugins/events-manager/templates/event-single.php

you should put them here:

/wp-content/themes/my-child-theme/events-manager/event-single.php

See this documentation page: https://wp-events-plugin.com/documentation/using-template-files/